This invention relates generally to photolithographic preparation of multilayer and/or high aspect ratio microstructures. More specifically, the invention relates to the preparation of such microstructures using electromagnetic radiation of multiple wavelengths.
xe2x80x9cNanotechnologyxe2x80x9d refers to nanometer-scale manufacturing processes, materials and devices, as associated with, for example, nanometer-scale lithography and nanometer-scale information storage. See, for example, Nanotechnology, ed. G. Timp (New York: Springer-Verlag, 1999), and Nanoparticles and Nanostructured Films, ed. J. H. Fendler (Weinheim, Germany: Wiley-VCH, 1998). Nanometer-scale components find utility in a wide variety of fields, particularly in the fabrication of microelectromechanical systems (commonly referred to as xe2x80x9cMEMSxe2x80x9d). Such systems include, for example, micro-sensors, micro-actuators, micro-instruments, micro-optics, and the like. Many MEMS fabrication processes exist, and tend to fall into the two categories of surface micro-machining and bulk-micromachining. The latter technique involves formation of microstructures by etching directly into a bulk material, typically using wet chemical etching or reactive ion etching (xe2x80x9cRIExe2x80x9d). Surface micro-machining involves fabrication of microelectromechanical systems from films deposited on the surface of a substrate, e.g., from thin layers of polysilicon deposited on a sacrificial layer of silicon dioxide present on a single crystal silicon substrate (this technique is commonly referred to as the xe2x80x9cthin film polysilicon processxe2x80x9d).
An exemplary surface micro-machining process is known as xe2x80x9cLIGA.xe2x80x9d See, for example, Becker et al. (1986), xe2x80x9cFabrication of Microstructures with High Aspect Ratios and Great Structural Heights by Synchrotron Radiation Lithography Galvanoforming, and Plastic Moulding (LIGA Process),xe2x80x9d Microelectronic Engineering 4(1):35-36; Ehrfeld et al. (1988), xe2x80x9c1988 LIGA Process: Sensor Construction Techniques via x-Ray Lithography,xe2x80x9d Tech. Digest from IEEE Solid-State Sensor and Actuator Workshop, Hilton Head, S.C.; Guckel et al. (1991) J. Micromech. Microeng. 1: 135-138. A related process is termed xe2x80x9cSLIGA,xe2x80x9d and refers to a LIGA process involving sacrificial layers. LIGA is the German acronym for X-ray lithography (xe2x80x9clithographicxe2x80x9d), electrodeposition (xe2x80x9cgalvanoformungxe2x80x9d) and molding (xe2x80x9cabformtechnikxe2x80x9d), and was developed in the mid-1970""s. LIGA involves deposition of a relatively thick layer of an X-ray resist on a substrate, e.g., metallized silicon, followed by exposure to high-energy X-ray radiation through an X-ray mask, and removal of the irradiated resist portions using a chemical developer. The mold so provided can be used to prepare structures having horizontal dimensionsxe2x80x94i.e., diametersxe2x80x94on the order of microns. The technique is now used to prepare metallic microcomponents by electroplating in the recesses (i.e., the developed regions) of the LIGA mold. See, e.g., U.S. Pat. No. 5,190,637 to Guckel et al. and U.S. Pat. No. 5,576,147 to Guckel et al.
Typically, complex three-dimensional microcomponents may be formed in part by successive application of LIGA or other lithographic techniques. First, a mold is prepared by depositing a layer of resist on a substrate, exposing the layer of resist to radiation through a patterned mask, and removing the resist layer according to the pattern. The mold is filled with a filler material, e.g., a metal, that will eventually become the three-dimensional component or a portion thereof. To ensure that no voids are included in the final microcomponent, the mold may be xe2x80x9coverfilled,xe2x80x9d i.e., excess filler material is used in filling the mold. Polishing is then carried out to expose a surface on which another mold may be formed by using the above-described process. By controlling the thickness of each resist layer and using an appropriate sequence of masks, a desired complex shape may be formed. However, this technique is not easily adaptable for large scale production because of the difficulty in carrying out the series of lithographic, filling and polishing steps, particularly in view of the size of such microcomponents.
There are a number of advantages associated with the use of deep X-ray lithography techniques such as LIGA in the preparation complex three-dimensional microcomponents. In addition, since X-ray photons are short wavelength particles, diffraction effects are absent for mask dimensions above 0.1 micrometer. Moreover, because X-ray photons are absorbed by atomic processes, standing wave problems, which can limit exposures of thick photoresist by electromagnetic radiation having long wavelengths are not problematic for X-ray exposures.
Ordinarily, LIGA requires a synchrotron that yields high flux densities, several watts per square centimeter, as the source of X-ray photons. Such sources generate X-rays with excellent collimation to produce thick photoresist exposures without any horizontal xe2x80x9crun-outxe2x80x9d as is described below. Locally exposed patterns therefore result in vertical photoresist walls if a developing system with very high selectivity between exposed and unexposed photoresist is available. However, the use of X-ray technology is also an inherent drawback in ordinary LIGA processes. For example, the dangers of X-ray exposure to living tissue are well known. In addition, X-ray technology and associated specialized equipment, X-ray masks and synchrotrons in particular, involve great cost due to the high-degree of expertise required to design, manufacture, operate and to maintain such equipment. For example, X-ray masks typically require hours to make while production of ordinary UV lithography masks require only a few minutes. Thus, there are strong economic incentives to find an alternative source or wavelength of electromagnetic radiation in order to carry out LIGA or other photolithographic processes for thick film applications.
One possible alternative to using X-ray technology is to employ ultraviolet-wavelength-based photolithography. Such photolithography is commonly practiced in semiconductor manufacturing processes and has been characterized. See, e.g., Flack et al. (1998), xe2x80x9cCharacterization of Ultra-thick Photoresists for MEMS Application s Using a 1xc3x97 Stepper,xe2x80x9d SPIE conference on Materials and Device Characterization in Micromachining, 3512:296-315, and Lxc3x6chel et al. (1994), xe2x80x9cGalvanoplated 3D Structures for Micro Systems,xe2x80x9d Microelectronic Engineering 23:455-459. Consequently, equipment associated with ultraviolet photolithography is much less expensive and much more commercially available than synchrotrons and other X-ray equipment as a whole. However, as a general rule, ordinary photolithographic techniques using ultraviolet radiation cannot achieve the resolution and the aspect ratios associated with ordinary LIGA process using X-ray technology.
Advanced photolithographic techniques have been proposed in semiconductor processing in order to achieve definition of fine lines with high aspect ratios. Such techniques may involve the use of multilayer resists as described in Ghandi, VLSI Fabrication Principles, Silicon and Gallium Arsenide 687-690 (2nd ed., John Wiley and Sons, 1994). This section of the textbook describes that there are two situations in which two layers of photoresist may be used. The first situation involves providing a first resist layer with high radiation sensitivity on a second resist layer having a low radiation sensitivity, exposing both resist layers to optical radiation and developing the layers. As described, this situation tends to result in significant undercutting to the thicker resist layer and is therefore unsuitable for most LIGA or other applications for producing microcomponents having high aspect ratios. The second situation also require two resist layers, a first resist layer sensitive to electromagnetic radiation of a first wavelength but opaque to a second wavelength and a second resist layer sensitive to electromagnetic radiation of the second wavelength. The first resist layer is applied on the second polymeric layer, and the first resist layer is exposed to radiation of the first wavelength and developed to define a pattern therein. The pattern is then used as a mask to expose the second resist layer to radiation of the second wavelength. The drawback of the second situation is that the first polymeric layer must be sufficiently thin to avoid standing wave, diffraction and other like problems. It is evident, then, that neither situation results in features having aspect ratios comparable to those using X-ray technology.
Thus, there is a need in the art for alternative methods that provide for microcomponents having high-aspect ratio features similar to those made by LIGA or other X-ray lithographic processes without use of X-ray. In addition, it is further evident that opportunities exist in the art for using multilayer and/or multiple wavelength lithography to enhance versatility of lithographic fabrication of complex three-dimensional microcomponents.
Accordingly, the invention is directed to the aforementioned need in the art and provides a method for preparing a multilayer microstructure using photodefinable compositions having different exposure wavelengths.
It is another object of the invention to provide such a method that allows greater flexibility in forming three-dimensional microstructures and microcomponents than ordinary LIGA or other X-rays technologies
It is a further object of the invention to provide such a method in order to form multilayer microstructures having a feature exhibiting an aspect ratio that exceeds the capacities predicted by aerial image calculations.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.
In a general aspect, the invention relates to a method for preparing a multilayer microstructure. The method involves applying a first photodefinable composition having a first exposure wavelength on a substrate to form a first polymeric layer and exposing a portion of the first photodefinable composition to electromagnetic radiation of the first exposure wavelength to form a first pattern in the first polymeric layer. Then, a second photodefinable composition having a second exposure wavelength is applied on the first polymeric layer to form a second polymeric layer. After a portion of the second photodefinable composition is exposed to electromagnetic radiation of the second exposure wavelength to form a second pattern in the second polymeric layer, a portion of each layer is removed according to the patterns to form a multilayer microstructure having a cavity having a shape that corresponds to the portion removed from the layers. Preferably, one of the first or second exposure wavelengths is an ultraviolet or visible wavelength. Optimally, both the first and the second exposure wavelengths are ultraviolet or visible wavelengths. Optionally, the multilayer microstructure is used as a mold in which the cavity is filled with a material that conforms to the shape of the cavity. By separating the material from the multilayer structure without substantially disturbing the material, the method may be adapted to form microcomponents or features of microcomponents having high aspect ratios as well as three-dimensional shapes.
In another aspect, the invention relates to a multilayer microstructure that comprises a substrate, a first polymeric layer on the substrate, a second polymeric layer on the first polymeric layer and a cavity extending through each polymeric layer and terminating at the substrate. The cavity has a substantially identical cavity cross-sectional area along the height of the cavity and an aspect ratio of at least approximately 30:1. The multilayer structure allows microcomponents or features thereof to be formed having a high aspect ratio without using X-ray lithography.